I'm thinking of what it will take to make host-model the default cpu mode for VMs in virt-manager. Right now there are still other issues with host-model, like it incorrectly modeling the host CPU, but this is another piece that I think is missing.
The host-model values are filled in every time the VM is run. This could potentially cause a problem on libvirt upgrade, if libvirt changed the logic for generating host-model, or a new CPU feature was added, or similar: the CPU definition would change across reboots, which could cause windows reactivation.
Possible solution is to do the host-model lookup only on the first VM run, and keep it forever. But not sure how that would work exactly.
Maybe this isn't an issue in practice or there's a better alternative...
Still an issue with latest code
This hasn't really turned out to be an issue any cares about in practice, so i don't think it needs explicit libvirt support. If apps care, they can grab the host-model CPU block from domcapabilities and define it into the XML themselves, which will accomplish the same goal